Recently, emission regulations on NOx contained in exhaust gas of diesel engines have been tightened. While gasoline engines broadly adopt three-way catalysts for purifying NOx, the three-way catalysts do not have a high purification performance unless engine combustion is conducted at a stoichiometric air-fuel ratio. Therefore, diesel engines, where engine combustion is conducted at a lean air-fuel ratio, do not adopt the three-way catalysts.
As an exhaust emission control system for such a diesel engine, it is known to use a NOx Storage Catalyst (NSC) for storing NOx and a urea Selective Catalytic Reduction (SCR) catalyst for detoxifying NOx by causing a chemical reaction of NOx with ammonia, in combination with each other. Since the urea SCR catalyst exhibits higher purification efficiency than that of the NSC within a high temperature range, by using the two catalysts in combination, NOx is efficiently purified over a wide range of temperature.
Although the storage ability of the NSC degrades as the NOx stored amount increases, it is restorable by bringing the air-fuel ratio of the exhaust gas close to (near) the stoichiometric air-fuel ratio or to be rich, and reducing the stored NOx (NOx reduction processing). The storage ability of the NSC also degrades when a catalyst temperature is low, e.g., immediately after an engine start (cold start). Therefore, for example, JP2008-121596A discloses a conventional exhaust emission control system in which, when a catalyst temperature and a storage ability are low immediately after the engine start, NOx reduction processing is performed as quickly as possible so as to secure the storage ability of the NSC.
However, when the NOx stored amount of the NSC is small, the NOx reduction processing performed immediately after the engine start does not sufficiently improve the storage ability, and since the NOx reduction processing requires bringing the air-fuel ratio of the exhaust gas close to the stoichiometric air-fuel ratio or to be rich, fuel consumption inconveniently increases.
Meanwhile, the urea SCR catalyst does not require NOx reduction processing. Therefore, with the exhaust emission control system using the NSC and the urea SCR catalyst in combination, by raising the catalyst temperature to the high temperature range where the purification efficiency of the urea SCR catalyst is high, the frequency of performing the NOx reduction processing immediately after the engine start is reduced. Thus, an increase in fuel consumption is prevented.